The invention relates to an apparatus for ultrasonic scanning comprising an ultrasonic applicator consisting of a plurality of ultrasonic transducer elements spatially adjacently arranged in a surface and a control device which effects a connection of a number of ultrasonic transducers, said number corresponding to the desired focal distance, to a signal transmitter or signal receiver, respectively, in accordance with predeterminable receiving--and/or transmitting--surface patterns.
In apparatus of this type, it is necessary that switching clicks (or transients) occurring, e.g. during the connection of individual transducer elements for the purpose of producing desired receiving or also transmitting surface patterns, be attenuated to a great extent or even eliminated from the transmitting or receiving lines. Particularly in the transmit mode, in order to prepare the transmitting patterns, the switching click of the switches must lie approximately 40 dB below the transmitting voltage to be subsequently transmitted (with a given impedance of the transducer elements). Similarly, however, in the receiving mode--i.e., during the switching of the receiving surface patterns, precaution must be exercised in order that those switching interferences; i.e., switching clicks which originate from the receiving switches, also lie beneath a predetermined threshold, Practice has shown that, particularly in the latter instance; i.e., in the receiving mode, the switching clicks occurring during switching-on of the receiving patterns act like small transmitting pulses which correspondingly produce echo signals in the body tissue to be examined. However, the latter echo signals then become undesirably superimposed, in the form of echoes proximate to the skin, on the echoes emanating from greater image depths which are to be represented. In the final analysis, this leads to image falsifications (ghost images).